Gel coats are widely used in numerous applications as the external surface layer of composite molded articles. Gel coats are typically found on composite articles that are exposed to the environment requiring moisture resistance, resistance to cracking and similar properties, or articles that require a strong, flexible, abrasion and impact resistant surface and/or a smooth glossy finish. Examples of such articles include boat hulls, bath tub enclosures, pools, spas, and body panels on cars and trucks, among others.
Gel coated articles are typically formed by spraying a gel coat composition from a high pressure spray gun onto the inside surface of an open mold, optionally applying one or more other materials, and applying a laminating resin for the composite article onto the gel coat or intermediate materials. The forming method typically includes curing the gel coat and then removing the cured gel coated article from the mold. Gel coated articles can also be fabricated by applying the composite materials into a multi-part mold, injecting or applying the gel coat composition, closing the mold, curing the gel coat and then removing the cured gel coated article from the mold.
Gel coats for composite articles are typically formulated from a thermosetting base resin system such as unsaturated polyester, acrylate and urethane type resins with incorporated fillers, pigments and other additives. The gel coat should exhibit low viscosity at high shear to allow for ease of application to the mold, but also resist sagging or running after it is applied. Another important property of gel coats is surface tackiness and cure time.
Typically, the gel coat resin is mixed with reactive, polymerizable monomers such as styrene or methyl methacrylate (MMA), which are also used to reduce resin system viscosity in order to apply the gel coat by spraying. Conventional gel coat compositions contain 35 to 45 wt % of reactive monomers and other volatile organic compounds (VOCs). The presence of high amounts of styrene and other VOCs results the emission of styrene vapors and other hazardous air pollutants (HAP), which are closely regulated by government regulations.
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